They burst onto the scene like lightning. Now they have scattered. Were they a boycott-created mirage, the gold medal-winning American men's gymnasts of the 1984 Olympics? A once-in-a-millennium-at-a-quadrennium cosmic fluke?

These were the relevant questions Sunday as a crowd of 6,633 in the Denver Coliseum watched a six-man U.S. men's team fall short of its Olympic heritage in the McDonald's Challenge, a head-to-head competition against the world champs from the U.S.S.R. The Soviet team beat the Americans by a hefty 5.85 points, 295.700-289.850, also hogging the medals in the individual competition. The Americans' brightest showing was provided by Dan Hayden, who tied for fourth place. He tried to turn the team's Rocky Mountain Low performance into a psychological upper. "I got to see the Russians before the world championships," Hayden said. "Now I know what I have to do to beat them."

If the Soviets' easy victory exposed the cracks in the U.S. men's gymnastics program, so, too, did a comparatively strong showing by the U.S. women. Although the U.S. women lost on Saturday by .625 of a point to the U.S.S.R. women, Kristie Phillips, 15, won this country's first all-around dual meet title against the Soviets in four years. While the Soviets were making major gaffes, Phillips nailed each of her routines to edge Natalia Laschenova 39.525-39.450.

The U.S. men desperately need a similar star to bolster their program. Yes, there has been cause for encouragement of late: Hayden and Scott Johnson finished one-two at the Cup of Excellence in Montreal last month; and Brian Ginsberg won the McDonald's American Cup in Fairfax, Va., in March. But in recent duels against the mettle-testing Soviets and Chinese, the team flopped, and the '85 world championships (ninth place) and the '86 Goodwill Games (fifth place) were unmitigated disasters. Said men's coach Abie Grossfeld on Friday, "I would like our guys to do a respectable job. I don't expect miracles."

Just how have the gymnasts tumbled from winning gold to losing respectability? First and foremost, it is necessary to remember that the Soviets were not in L.A. in '84 and that things might have been different had they been there. Further, the nation's top three gymnasts retired shortly after those games. Peter Vidmar took his silver-medal steadiness into assorted business ventures, Bart Conner parlayed his three Olympics' worth of experience into work as a TV commentator, and the groupie-pleasing Mitch Gaylord went off to become an actor. This American troika, along with Jim Hartung, who is also retired, had ruled the sport here since banding together after the U.S. boycott of the Moscow Olympics in 1980.

Intensity was never a problem for the '84 U.S. Olympians. Spurred by the '80 boycott, they frequently went through their routines in practice as if a gold medal were on the line. "In '84 it was an 'Eye of the Tiger' concept," says Robert Cowan, the men's program director for the U.S. Gymnastics Federation. "Now it's an 'Eye of the Hamster' type of deal. But we're getting there."

Against the 1984 team's high standards of deportment, the current crop, which has laughed on live TV after a fall and brought beers into interview rooms, hardly measures up. "This team doesn't have the personal bonds and doesn't work as a unit like the '84 team," says Mike Jacki, the USGF executive director. Far from laughing in Denver, the Americans sat nervously beneath the stands before the competition began, staring vacantly, and when they were introduced by the P.A. announcer, they were strangely dispirited. By contrast, the Soviets playfully straightened the collars of teammates' warmups and behaved as if their only care in the world was where they would go shopping afterward in this Land of Levi's.

On top of everything else, the American men have had injuries to contend with. Dennis Hayden, 22, Dan's identical twin, is recovering from knee surgery; and Tim Daggett, the only individual medalist from L.A. still competing, landed on his head during practice in February while attempting a flip catch from the high bar. He was in traction for 10 days and only recently removed his cervical collar.

Showing no mercy, coach Leonid Arkaev had brought along his finest. "We could have [brought others here]," he said, Arkaev paused. Then he smiled. "We have a very good junior team as well." He smiled again.

The Soviets were, in a word, overwhelming. They featured the brilliance of winner Valeri Lyukin, 21, whose acute spatial sense is taking Soviet gymnastics to a higher level of virtuosity; the technical expertise of two-time world champ Yuri Korolev, the runner-up; and the brutish power of third-place finisher Valentin Mogilny.

Hayden's relative success against such daunting competition may have something to do with his background, which in some ways approximates that of his Soviet rivals. His dedication, despite serious back injuries dating back to 1981, is unquestioned. Like the Soviets, he benefited from continuity of training, working since 1980 with Yoichi Tomita in Tucson at one of the U.S.'s few all-male gymnastics centers. In the face of threatened NCAA cutbacks in the amount of time athletes will be allowed to train and with the wavering commitment to compulsories in college gymnastics, Hayden has left Arizona State to focus on gymnastics.

But most important, Hayden, like Johnson and Ginsberg, has routines that compare in difficulty with those of the Soviets. In a high bar routine that had Arkaev's total attention, Hayden executed a double overcatch (a "Kovacs," so called after the gymnast who first executed it) and a full-twisting double layout dismount over the bar (perhaps soon to be called a Hayden, because he's the only one who does it) for a 9.90. "The key is to always improve in difficulty," Jacki says. "That's where the Soviets always have the advantage."

But there are other areas in which the Soviets have the edge as well. The U.S. system can't guarantee the participation of athletes like Hayden, nor can it carefully groom their successors. Tomita believes that the U.S. naturally spawns far more men's gymnasts than any other country but that, because of the number of distractions and alternative opportunities in this country, concentration, cohesion and coaching regimens are often disrupted.

"Every 10 or 20 years here you get a team like the one in '84," Tomita says. "Those are exceptions. A national program cannot depend on exceptions."

PHOTORICH CLARKSON

Hayden, who tied for fourth place, his team's highest finish, is now a full-time gymnast.